(1) Field of the Invention
This invention relates to a method for diagnosing the interior of a rotary kiln employed for firing pellets of iron ore, calcining lime, sintering chromium ore or the like, and especially to a method for diagnosing the interior of such a kiln as mentioned above which method permits to make a correct determination as to the state of deposit of ash which occurs as a problem when pulverized coal is used as a indurating heat source.
The present invention will be described with the induration work of pellets of iron ore as its principal application field, but the present invention can be equally utilized in a variety of other field such as lime-calcining kilns.
(2) Description of the Prior Art
As indurating methods of iron ore pellets, there have been known, roughly breaking down, three different types of systems which are the shaft kiln system, the travelling grate system and the grate-kiln system. Among these systems, it is the grate-kiln system or its improved model that the present invention pertains to. According to the grate-kiln system, the drying and preheating (hereinafter called merely "preheating") of green pellets is carried out on a travelling grate and, after indurating the thus-preheated pellets completely in a rotary kiln, the resultant indurated pellets are then cooled in a cooling apparatus which is generally called an annular cooled. These three processing steps are allotted to different facilities respectively but these facilities are closely connected and operated practically as a single unit. For example, the heating of the travelling grate is effected in its entirety by means of burners only, which burners are provided at the discharge end of the rotary kiln. Thus, the travelling grate unit (i.e., the travelling grate) is not provided with any rows of burners and its heating is dependent on a high-temperature gas supplied from the rotary kiln. This system enjoys such merits that pellets are subjected to little disintegration thereby assuring a high production yield and a uniform firing operation, because green pellets are kept in a stationary state on pallets during their preheating periods, in which the strength of green pelllets becomes lowest, and are indurated in the rotary kiln while caused to undergo cascading therein. Besides, the heat of off gas from the rotary kiln is used as a heat source for the preheating as is and the low heat efficiency, which is considered to be a common defect of general rotary kiln systems, does not become a problem from the practical viewpoint of the grate-kiln system.
Although the grate-kiln system enjoys such merits as mentioned above, it still involves as an unsolved problem the problem common to the rotary kiln system, namely, the occurrence of rings. Therefore, rings must be removed at every scheduled shut-down which is carried out rather often. Such rings tend to develop intensively, especially, at two locations of a rotary kiln, i.e., at the feed end and an high temperature zone of the rotary kiln. As causes for the development of such rings, matter pertaining heat, heat properties of charged raw materials, the quality and quantity of iron ore dust resulting from the raw materials and the like are considered to interact, although their method of interaction has not yet been elucidated completely. On the other hand, it has become an important subject in the present field of art to regard coal as a fuel in the light of various changes surrounding oil, resulting in an change in the fuel injected from burners provided at the discharge end of each rotary kiln, or other words, leading to the conversion to pulverized coal. However, pulverized coal fuel contains a great deal of ash derived from coal and tends to promote the development of the above-mentioned rings. Thus, some coal-fired rotary kilns become unable to continue operations before their next scheduled shut-down.
It is also known that occurrence of such rings takes place in various ways or fashions depending on the type of coal and as fuel materials (place of mine and kind of coal). According to a report on "A Study of Coal Firing in the Grate-Kiln System" (addressed at the 50th AIME Annual Meeting held in January, 1977), it is mentioned that use of coal having a DP (Deposition Parameter) value over 300, which DP value is given in accordance with the following empirical equation: ##EQU1## where A means the percentage of ash and H.sub.v stands for the net heat value (unit: British Thermal Unit/lbs.) and indicates the susceptibility of deposition of ash and its analogous substances at the feed-end of a rotary kiln, is not recommendable because deposition of ash becomes severe at the feed-end of the rotary kiln.
Furthermore, the susceptibility of ash deposition at a high temperature zone of a rotary kiln is represented by RP (Ringing Parameter). Any coals having RP values exceeding 150, which RP values are given by the following experimental equation: ##EQU2## where F.multidot.T means the fluid temperature of the ash in an oxidizing atmosphere, expressed in terms of .degree.F., are said to be unrecommendable as they lead to considerable deposition of ash at the aforementioned high temperature zone. When using pulverized coal as a fuel, it is recommendable to determine DP and RP whenever the coal is changed from one supply source to another and to conduct the operation by principally using coals which satisfy DP.ltoreq.300 and RP.ltoreq.150. However, this operation method permits to use only coals which practically meet the above reference values. Accordingly, the above operation method is not only unable to contribute to the reduction of production cost but is also extremely poor in adaptability to situations as it cannot provide any countermeasure even if a change in the state of deposition of ash is observed following a change in operational conditions.